1. Field of the Invention
The present invention relates to a board recording apparatus capable of displaying an image such as a manuscript on a vertical plane like a blackboard. More particularly the invention concerns the elimination of smudges caused by previously displayed images on a board recording apparatus.
2. Description of the Related Art
The following is a description of a previously proposed board recording apparatus. As shown in FIG. 6, a board recording apparatus has a recording medium 102 in the form of an endless belt looped around a pair of vertically displaced rollers 100 and 101. An image formation apparatus 103 is disposed ad]acent the outer surface of the belt near the lower roller 101. This image formation apparatus is disclosed in Japanese Patent Publication No. 51-46707 (U.S. Pat. No. 3,914,771) and others; and it is constructed such that an image is produced by applying voltages corresponding to image signals from a recording control portion 105 to a recording electrode 104 and depositing toner onto the thus charged surface of the recording medium 102. The recording medium 102 on which the image is produced, is moved to a display area 106 and displayed. Thereafter, the deposited toner 107 is scraped and removed from the recording medium 102 by a cleaning means 108, to enable a subsequent display on the recording medium.
The above described board recording apparatus has certain problems. As can be seen from FIG. 7A, the image recording process carried out in the apparatus of FIG. 6 comprises the steps of applying predetermined positive voltages to the recording electrode 104, grounding a conductive layer on the back of the recording medium 102, and allowing the toner 107 to become charged to positive polarity so that it becomes deposited on the recording medium 102 as a result of electrostatic forces existing across the thickness of an insulating layer on the front of the recording medium. The toner 107 deposited on the surface of the insulating layer is thereafter scraped and removed by the cleaning means 108. However, if the insulating layer is a dielectric which has a high electrical resistivity or time constant, then the electrostatic charges from the toner 107 do not leak away fully from the insulating layer but instead remain as residual charges 111 (FIG. 7B). When the next image is developed in this circumstance, as shown in FIG. 7C, there arises a problem of smudge in the image on the recording medium 102 because minus charges are induced in the toner due to the residual plus charges in the insulating layer 110; and as a result, toner with a minus charge is deposited on the recording medium 102 in addition to the toner deposited with a plus charge according to a new image. In order to solve this problem, it has previously been proposed to reduce the electrical resistivity of the insulating layer 110. However, this gives rise to another problem, namely an image of low density. This is because when the resistivity of the insulating layer 110 is low, the charges on the toner 107 leak away through the insulating layer 110 and it then becomes difficult to deposit toner on that layer.